This invention relates to permanent magnet rotors and, more particularly, to rotors with wedge-shaped permanent magnets and cushions between the magnets and their support members.
A permanent magnet generator generally consists of a rotor including an even number of equally spaced, alternating polarity magnets around the radial periphery and a stator which includes a number of windings arranged to obtain magnetic coupling with the rotor poles. Rotation of the rotor causes the flux linkage of the permanent magnets with respect to the stator coils to vary, thus inducing an electromotive force in each of the stator coils.
In the prior art, the rotor permanent magnets have been rectangularly shaped and supported against centrifugal load by a hoop shrunk over the rotor's periphery. The magnets have relatively poor tensile stress but fairly good compressive strength so they should be compressively retained. In the case of high speed rotors, the thickness of the magnet retaining hoop greatly increased, disproportionate to other parts of the rotor, because self-stress of the hoop was large and little structural capacity remained available to carry the magnets. Also, difficulties arose in uniformly distributing the magnet load into the hoop, and this introduced bending stress concentrations into the hoop. Also, as the hoop thickness was increased, reluctance of the magnetic circuit increased and pole head leakage increased. Since magnets generally have very little ductility and tend to be fragile, another problem encountered by the permanent magnets at high speeds was that they would fracture due to excessive stress concentrations at their support surfaces.